The present invention relates to a method for reducing variations in the bending of rolled metal base plates for semiconductor modules; the present invention also relates to a semiconductor module having a semiconductor component which is applied to a base plate that is provided with a heat sink.
As is known, the power loss of semiconductor modules and, in particular, power semiconductor modules, for example IGBTs (Insulated Gate Bipolar Transistor), power transistors, thyristors and so on, is dissipated, via a metal base plate, to a heat sink that is mechanically connected to the base plate. Suitable materials for such base plates are, in particular, copper and aluminum. However, other materials such as alloys and so on are also possible in this case.
A gap is often produced between the base plate and the surface of the heat sink on account of unevennesses or the like. However, this gap should be as small as possible in order to improve the thermal conductivity between the base plate and the heat sink.
A screw connection is one customary possible way of fastening a base plate to a heat sink. That is to say that edge of the base plate which projects beyond the semiconductor component is fastened to the top side of the heat sink by means of screws.
Base plates between the fastening points, that is to say the locations having the screw connections, now usually have a convex shape: they thus have a “positive” bend, so that they are pressed onto the top side of the heat sink when screwed.
If additionally metalized insulating ceramics are soldered, for example, to the top side of the base plate between the latter and the underside of the semiconductor component, a bimetal effect results on account of the different coefficients of thermal expansion of the base plate, on the one hand, and the insulating ceramic, on the other hand. This means that, on account of this bimetal effect, the bending of the base plate during the soldering process is changed when applying the insulating ceramic, considerable variations in this bending occurring.
For the purpose of illustration, FIG. 1 illustrates a semiconductor module 1 including a semiconductor component 2, for example an IGBT having connections 3. The semiconductor component 2 is firmly fitted, via a copper base plate 4, to the top side 5 of a heat sink 6 (having cooling ribs 7) using screws 8. The bending of the base plate 4 can be clearly seen in FIG. 1. This bending is eliminated or compensated for by the screws 8 being screwed into the top side 5 of the heat sink 6 so that the underside of the base plate 4 completely rests on the top side 5 of the heat sink 6.
In order to compensate for the variation in the bending of the base plates 4, the variation being produced by the bimetal effect mentioned, the bending of the base plates is selected, on average, to be larger than is required per se. Greater bending of the individual base plates is thus deliberately accepted in order to compensate for variations in the magnitude of bending. However, such greater bending increases the risk of the ceramic breaking when mounting the module if, in particular, the base plate 4 is screwed to the heat sink 6.
FIG. 2 illustrates a plan view of a base plate 4 for an IGBT module. This base plate has openings 9 for the screws 8 in its longitudinal direction B. The bending in the transverse direction A of the base plate 4 should have as little variation as possible since, in this case, the distance a between the holes 9 for the screws 8 is considerably greater than the distance b between the openings 9 in the longitudinal direction B. In other words, it can be seen in FIG. 2 that the variation in the bending in the longitudinal direction is of lower importance than in the transverse direction since the base plate is pressed onto the top side 5 of the heat sink 6 at short intervals in the longitudinal direction B using the screws 8. Conditions where a is quite generally large in comparison with b are generally present per se in base plates.
As is known, base plates for semiconductor modules are preferably produced by rolling copper sheet, for example. This copper sheet is then punched after rolling, so that the individual base plates are punched from the rolled sheet.
For these and other reasons, there is a need for the present invention.